Between the two, the difference in x is -1 and the difference in y is 3.
Y's difference is rise, and x's difference is run.
Divide rise over run to get the slope.
3 / -1 = -3
So the slope is -3, eliminating 2 and 4.
The y-intercept that fits the graph is 7, not -7.
That means the correct equation is C.
Hope this helps!